Conventionally, vinylpyrrolidone polymers such as polyvinylpyrrolidone and vinylpyrrolidone copolymer have widely been used in various uses such as medicines, cosmetics, pressure-sensitive or bonding adhesives, coatings, dispersants, inks and electronic parts by reason of having merits and advantages such as biocompatibility, safety and hydrophilic property. On that occasion, the molecular weight of vinylpyrrolidone polymers to be used has an optimum range in accordance with each of the uses; for example, the polymers having low molecular weight, namely, the polymers which are low in K value determined by a Fikentscher's method (occasionally referred to as simply “a K value” hereinafter) are suitable in uses such as pressure-sensitive or bonding adhesives for glue stick, cosmetic materials such as humectants and hair conditioners, and tablet binders.
Incidentally, in the case of using vinylpyrrolidone polymers for various uses, in order that solvent used in polymerization for obtaining the polymers may not remain in the polymers, or in consideration of convenience during transportation and storage, the polymers are once formed into a powdery state, thereafter dissolved in desirable solvent and used as solution in many cases.
The followings are generally adopted for industrially obtaining a powder of a vinylpyrrolidone polymer:
i) a process of drying a solution containing a vinylpyrrolidone polymer (typically, a polymerization solution obtained in polymerization for synthesizing a vinylpyrrolidone polymer) by a spray-type dryer such as a spray dryer to obtain a powder, and
ii) a process of drying a solution containing a vinylpyrrolidone polymer (typically, a polymerization solution obtained in polymerization for synthesizing a vinylpyrrolidone polymer) by hot surface adhesion-type dryers such as a disk rotary dryer, a drum rotary dryer and a belt dryer to form a dry matter in sheet shape or thin chip shape and thereafter pulverize the dry matter; and
the K value of a vinylpyrrolidone polymer has determined either i) or ii) to be adopted so far.
Specifically, above-mentioned process i) has generally been adopted in the case of a low K value, while the above-mentioned process ii) has generally been adopted in the case of a high K value (refer to Japanese Unexamined Patent Publication Nos. 2002-146033 and 62-62804). The reason therefor is that industrially, typically, a polymerization solution obtained in polymerization for synthesizing a vinylpyrrolidone polymer is directly used as a solution containing a vinylpyrrolidone polymer and dried; then, in the case of a vinylpyrrolidone polymer having a low K value, namely, a low molecular weight, for the reason that the viscosity of the solution to be dried is low, hot surface adhesion-type dryers such as a disk rotary dryer, a drum rotary dryer and a belt dryer do not successfully allow the dried polymer to be formed into a dry matter in sheet shape or thin chip shape and thereby are difficult to apply, while in the case of a vinylpyrrolidone polymer having a high K value, namely, a high molecular weight, for the reason that the viscosity of the solution to be dried is high, a spray-type dryer such as a spray dryer does not successfully allow the solution to be sprayed and thereby are difficult to apply. An example such that solution of a vinylpyrrolidone polymer having a low K value, for example, smaller than 50 is dried by hot surface adhesion-type dryers such as a disk rotary dryer, a drum rotary dryer and a belt dryer has not been reported.
However, a powder obtained by the above-mentioned process i) of drying by a spray-type dryer to be pulverized has problems that dust rises easily and that handleability is poor when the powder is dissolved for use in a desirable solvent into solution since most of the powder inevitably becomes fine particles having a small particle diameter. In addition, it is generally conceived that the smaller the particle diameter is, the more easily a powder is dissolved; contrary to this common assumption, a powder obtained by the above-mentioned process i) of drying by a spray-type dryer to be pulverized is very poor in solubility and has problems such that very intricate handling and long-time dissolution handling become necessary when dissolved for use in desirable solvent into solution. As described above, a powder of a vinylpyrrolidone polymer having a low K value has conventionally been obtained by drying by a spray-type dryer of the above-mentioned i), and a powder of a vinylpyrrolidone polymer having a low K value, which can solve the problems of handleability and solubility when dissolved in the above-mentioned solvent, has not been known.
Then, the problems to be solved by the present invention are to provide a powder of a vinylpyrrolidone polymer, which has excellent solubility in spite of having a low K value determined by a Fikentscher's method and can be made into solution with favorable handleability without dust rising, and to provide a process for production of the same.